"Sherlock Holmes-style detective, Pablo, travels to Mystery Manor to find out who stole Lady Tasha's jewels. Among the suspects are Butler Tyrone, Mr. Austin Frothingslosh, and Inspector Uniqua Underhood. It's a rousing game of cat and mouse through secret passageways and mazes to discover…whodunit!"
Pablo exits his house wearing a detective's hat. He tells the viewer that he is the greatest detective in the world. He sings the song "Aha!" and notices a mysterious card near a tree. He reads it aloud and explains that the note is from Lady Tasha, telling him that there is a mystery afoot. The backyard transforms into a misty Scotland scene.
Pablo walks across the landscape, but when he stops, footstep noises continue. He thinks someone is following him and sees a muffled figure standing in front of a hedge maze. He asks the figure where Mystery Manor is, and the figure points to a mansion. Pablo looks at the building for a second and turns back to thank the mysterious person, but the figure is gone.
He knocks on the door and is greeted by Butler Tyrone, who leads Pablo to Lady Tasha's room. Pablo meets her and Mr. Austin Frothingslosh, the next-door neighbor. He asks the two if they have noticed mysterious things happening while singing "Mysterious, Very Mysterious". A figure enters the room, claiming to be Miss Uniqua Underhood from London. She tells Tasha that someone will come to take her jewels.
Pablo asks how she knows the information, but is interrupted when the lights go off. Screams, glass breaking, and hurrying footsteps are heard. When the lights come back on, Tasha faints because her jewels are gone. Uniqua and Pablo see a mysterious figure running outside. They know that it must be the jewel thief. Since Butler Tyrone is not currently in the room, the two predict that the figure was Tyrone.
They hear muffled sounds coming from the closet and open the door. Tyrone comes out and explains that the jewel thief pushed him into the closet. Austin says that he needs to leave because he needs to feed his cat. Pablo thinks that he invented that story for just leave.
Pablo and the others go back to Lady Tasha's room to talk about the mystery. Tyrone spots a mysterious figure at the window. Uniqua and Pablo run after it, but the strange figure has entered the garden. Uniqua and Pablo enter the garden and sing "The Fleeting Phantom Figure in the Garden" as they separate. Pablo catches Austin and orders him to give him Lady Tasha's jewels, but he claims that he never had them. Austin explains that he was walking to the mansion after feeding his cat.
Pablo and Austin spot two mysterious figures entering the mansion. They enter and catch Miss Uniqua Underhood. Pablo orders her to hand over Lady Tasha's jewels, but she reveals that she is a policewoman and never took them. She says that she received a mysterious letter telling her that someone was planning to take Lady Tasha's jewels. She recalls a mysterious figure that she followed into the mansion. Uniqua, Pablo, and Austin spot footprints leading to the library and enter it.
Pablo finds a secret passage behind a bookcase. He tells Austin to stay with Lady Tasha, but talks slower as he sees Tasha snoring loudly on the couch. Uniqua and Pablo sing "Whodunit?" as they track down the thief. They find him in the hall closet and unmask him.
The thief is revealed to be Butler Tyrone. Pablo asks him why he took Lady Tasha's jewels. Tyrone states that Tasha gave them to him. Lady Tasha explains that Tyrone and her were bored and decided to invite their friends over for a mystery. Pablo exclaims, "What a great idea!"
Austin invites the others to his house for cheese and crackers. The mansion transforms back into the backyard as the children sing the end song. They walk to Austin's house through the door in the wooden fence. As the episode ends, Pablo then opens the previously closed door and shouts "Aha!"